1. Field of the Invention
The present invention relates to a lens system and an optical device using the same and, more particularly, is suitably applicable to optical devices such as silver-film cameras, video cameras, electronic still cameras, and so on with high optical performance, which lens system is well corrected for aberration variations during focusing in a wide subject range from an object at infinity to an object at a near distance.
2. Related Background Art
Conventionally, there are lens systems called macrolenses or microlenses (which will be called hereinafter “macrolenses” together) intended for the principal purpose of taking pictures of near objects with optical devices such as photographic cameras, video cameras, video still cameras, and so on.
The macrolenses are designed to yield high optical performance, particularly, for near objects, as compared with other taking lenses such as ordinary standard lenses, telephoto lenses, and so on.
In general, the macrolenses increase, particularly, longitudinal (axial) chromatic aberration and lateral chromatic aberration (chromatic aberration of magnification) among various aberrations with increase in the photographic magnification and it becomes difficult to correct them well.
Even if the lens systems are corrected well for spherical aberration, coma, astigmatism, etc. among the various aberrations, good optical performance cannot be expected unless the axial chromatic aberration and chromatic aberration of magnification are corrected well.
There are conventional methods of reducing chromatic aberration of optics by provision of a diffracting optical element with diffraction action on a lens surface or in part of the optics, making use of the physical phenomenon that a refractive surface and a diffractive surface in optics have reverse chromatic aberrations for rays of a certain reference wavelength, which are disclosed, for example, in documents such as SPIE Vol. 1354 International Lens Design Conference (1990) and the like, Japanese Patent Applications Laid-Open No. 4-213421 and No. 6-324262, U.S. Pat. No. 5,044,706, and so on.
The diffracting optical elements disclosed in these references utilize the configuration wherein a diffraction grating of periodic structure is arranged rotationally symmetric about a certain axis, e.g., the optical axis and periodic pitches of the diffraction grating are gradually changed whereby the diffraction action brought about by the ring structure of the periodic pattern acts as a lens.
These diffracting optical elements exhibit a great correction effect, particularly, for chromatic aberration appearing at a refractive surface because of dispersion of glass, and can be provided with an effect like an aspheric lens by changing periods of the periodic pattern, thus achieving a great effect of reducing aberration.
Specific structures of the diffracting optical elements for obtaining this diffraction action are called kinoform and there are well-known kinoform structures, e.g., those having continous spacings between portions with phase difference of 2π, those having binary shape (step shape) wherein a continuous phase difference distribution is approximated to a step shape, those wherein the microscopic periodic pattern is approximated to a triangular wave shape, and so on.
Such diffracting optical elements are fabricated by semiconductor processes such as lithography and the like, by cutting, or the like.
Particularly, the diffracting optical elements of the step shape (binary shape) among such diffracting optical elements are now fabricated readily with very high resolution and high accuracy by the semiconductor processes of lithography and the like.
In general, if the photographic magnification range is expanded in the macrolenses, particularly, toward higher magnifications, there will appear more aberration with change in the photographic magnification and it will become difficult to correct it well.
For example, if a macrolens designed on the basis of the photographic magnification of 1/10 is intended to expand the photographic magnification toward a high magnification of 1× and to take a picture, it will produce extremely great spherical aberration, curvature of field, coma, chromatic aberration, and so on.